Acoustic multi-mode logging device adapted to decouple noise within a semi-rigid receiver array

ABSTRACT

A multi-mode receiver sonde having individual receiver stations within a semi-rigid receiver array. The semi-rigid receiver array is adapted to decouple noise (tool-mode noise and/or road-noise) at one location within the receiver array from other locations within the receiver array by using compliant spacers between the receiver stations to absorb and scatter the noise. 
     In preferred embodiments, multi-mode receiver stations are presented which provide a common ground for output signals detected by the receiver stations. Additional embodiments provide for combining the output signals into composite signals which represent a selected borehole propagation mode. 
     In one highly preferred embodiment, the use of transducer detectors which have a flat frequency response over a selected bandwidth allows for accurate comparisons between composite signals so that changes in phase and amplitude over distance and time can be determined.

FIELD OF THE INVENTION

This invention relates to acoustic well logging in general and moreparticularly, to reducing noise in multi-mode acoustic well loggingreceivers.

BACKGROUND OF THE INVENTION

Conventional acoustic logging of earth formations traversed by aliquid-filled borehole is accomplished by lowering into the borehole alogging tool suspended on an armored communication cable. The typicallogging tool will usually incorporate several acoustic transducers. Atleast one transducer will be used as a transmitter to generate acousticsignals which are to be detected by one or more transducers that act asreceivers. The desired detected signals would be representative of theenergy from the transmitter which travels through the borehole or thesurrounding formation to the detector and it would not contain anythingelse, such as, a "tool-mode noise" or a "road noise" which will bediscussed hereinafter.

The acoustic signal generated by the transmitter centered in theborehole can be a symmetrical or an asymmetrical compressional waveformwith respect to the borehole axis in the fluid. When the generatedcompressional wave travels through the liquid in the borehole andstrikes the borehole wall, various types of elastic and guided waves,which will be referred to herein as borehole waves, are produced as theearth--borehole response to the generated signals. The types of boreholewaves produced have different velocities and amplitude--frequencycharacteristics. Since these borehole waves are usually detected at areceiver transducer through a fluid coupling, borehole waves will alsobe referred to herein as acoustic waves or signals. Because theseacoustic wave types have different velocities and characteristics,various methods are used to enhance the ability of the logging tool todetect the waveforms so that the wave types may be distinguished duringprocessing. Through the processing of these waveforms, particularlythrough the amplitude and phase relationships of the waveforms as afunction of time and distance, the viscoelastic properties of the earthformations surrounding the borehole can be deduced, such as, inparticular, the compressional and shear wave speeds of the earthformations.

Transducers used as receiver elements (also referred to herein astransducer detectors) may be combined to form a receiver station. Onetype of transducer, a piezoelectric transducer, has been used for thereceiver element. Prior art (as one example, U.S. Pat. No. 4,649,526)has taught: the use of multipole logging tools in subsurfaceexploration, the addition/subtraction of the output of a plurality ofdetector transducers to form one combined signal (a composite signal)for each receiver station, and the use of piezoelectric elements fordetector transducers. The combination of outputs can take several formsor modes. The selective addition and/or subtraction of signals to formthe composite signals from specifically located piezoelectric detectorelements at each receiver station is used to detect variousborehole-propagation modes. When used in this manner, the followingmodes of borehole propagation can be detected: monopole, dipole,quadrupole, octopole and other borehole-propagation modes which may beinitiated by selected multipole transmitters.

A logging tool generally consists of several receiver stations, spacedat some interval along the body of the tool. The collection of receiverstations will be referred to herein as a receiver array.

Preferably, the data collected by the receiver will only include anaccurate representation of the earth (or the borehole) response to thesignals generated from the transmitters.

However, signals obtained from conventional acoustic multipole loggingtools are subject to various noises such as "tool-mode noise" and "roadnoise" and they may also be affected by transducer detector resonanceeffects. Either of these problems limits the ability of the tools toobtain signals from the detector transducers which accurately and onlyrepresent the response of the borehole environment to the variouspropagation modes of the generated signals.

TRANSDUCER RESONANCE, AMPLITUDE AND PHASE DISTORTION

Transducers, and in particular, piezoelectric detectors, have resonantfrequencies which are dependent upon the type of material used to makethe transducer and the desired mode of operation. The deleteriouseffects of resonant frequencies which we discuss herein will apply toall and every resonances of a transducer; therefore, for reasons whichwill become obvious hereinafter, the resonant frequency discussed hereinis assumed to be the lowest or fundamental resonant frequency on atransducer frequency response curve that exhibits the effects ofresonance for a given material which is configured in a particular modeof transducer operation. The resonant frequency is determined by thedetector geometry, e.g., the shape, length, width, thickness andorientation of the poling axis within the material, for examples seeElectronics Engineering Handbook, Third Edition; edited by Fink andChristiansen, Poled Ferroelectric Devices by Giannotto, pages 7-19through 7-21, published by McGraw Hill Book Company, 1989.

A piezoelectric detector will provide an electrical output responsesignal when subjected to mechanical stress variations across the polingaxis. An acoustic field may provide the mechanical stress to thedetector. The desired output response from the detector is an electricalsignal which represents a component of the variations in the acousticfield (the acoustic signal) in the borehole. In this specification, themechanical stress variation placed upon the detector is called an inputsignal.

FIGS. 1A and 1B illustrate representative piezoelectric transducercharacteristic frequency response curves for a transducer having a twopole response with a single resonant frequency. They illustrate theeffect of a resonant-response region on the output response signal ofthe detector with respect to the input signal. The horizontal axis ofeach response curve is a logarithmic frequency scale starting at 100 Hzand ending at 100 kHz. The vertical axis of FIG. 1A shows the ratio ofthe output response signal amplitude to the input signal amplitude indecibels (dB) over the frequency range with the assumption that theamplitude of the input signal is constant across the frequency spectrum.FIG. 1A shows the resonant frequency for this detector to be atapproximately 10 kHz. The resonant-response region defined here as thefrequency region where the amplitude ratio is larger than someprescribed value of this detector is around 3 kHz to 13 kHz.

A detector transducer has higher sensitivity to input signals within aresonant-response region because input signals within this resonanceregion give output signals which are magnified by the resonance effect,compared to input signals of the same amplitude outside the resonanceregion. If the frequencies of the input signal for the transducerdetector in FIG. 1A are above 13 kHz, the output signal isproportionally smaller than the response to the input signals within theresonant-response region, and the higher the frequency of the inputsignal, the smaller the output response signal.

Using detectors which provide a greater response to input signals withinthe resonant-response region has the advantage of reducing thecomplexity of the receiver electronics, allowing the use of electroniccomponents which do not have to have the sensitivity that would berequired if the resonant-response region were not used.

In spite of the substantial advantages of using acoustic tools whichtake advantage of the resonant-response region of transducer detectors,there are some disadvantages. The output response signal is subject toundesirable frequency response characteristics in amplitude and phasebetween the input acoustic signal to the detector and the outputresponse signal from the detector. These undesirable characteristicswill be referred to herein as "amplitude distortion" and "phasedistortion".

Returning to FIG. 1A, it shows that within the resonant-response region,the amplitude of an output response signal to a constant input signalwill vary with respect to how close the spectral components of thesignal are to the resonant frequency; consequently, there is amplitudedistortion in the output response signal in relationship to the inputsignal. Whereas, if the output response signal had spectral componentswithin the frequency range of 100 Hz to approximately 3 kHz, thedetector provides a flat amplitude response over this frequency rangebecause the relationship between the amplitude of the output signal andthe input signal is approximately a constant ratio. This constantamplitude ratio region will be referred to herein as a flat amplituderesponse region.

FIG. 1B shows on the vertical axis, the phase difference between theoutput response signal and the constant input signal in degrees. At theresonant frequency, the phase shift (or phase difference) between theinput acoustic signal and the output response signal is 90 degrees.Across the resonant-response region of FIG. 1B, the phase shift couldvary as much as 180 degrees. Whereas, between 100 Hz and approximately 3kHz there is a region where there is significantly little phasedifference between the input signal and the output signal. Thisapproximately constant phase-difference output response region will bereferred to herein as a flat phase response region.

The flat amplitude response region and the flat phase response regionwill be referred to collectively herein as a flat frequency responseregion.

When detector stations are used for detecting signals within theresonant-response region, the composite signals may be degraded due toamplitude and phase distortion if the resonant-response region and otherfrequency response characteristics of each element of the receiverstations are not identical.

For example, a dipole borehole wave produces an acoustic field in theborehole which is antisymmetric (i.e. equal amplitude and a phasedifference of 180°) upon refection through the borehole axis. A dipolereceiver station could be composed of two transducers detectors locatedon diametrically opposite sides of the centered logging tool. If theresonant-response regions of the two transducers were identical, thensumming the output responses of both transducers would yield a nullresponse at all frequencies since the input dipole signal isantisymmetric, while differencing 15 the signals received at the twotransducers and halving the results would yield a true measurement ofthe amplitude spectrum of the acoustic fields in the borehole. However,if the resonance-response regions were not identical, summing and/ordifferencing the responses of the transducers would give an erroneousrepresentation of the acoustic signal in the borehole.

It is not possible to obtain transducers with exactly matchedresonant-response regions because in a downhole environment where thetemperature can approach 200 degrees Centigrade and the pressure canexceed 10,000 psi (68,940,000 Pascal); small differences in transducersof conducting and material properties, especially, piezoelectrictransducers, can result in differences in the resonant frequencies forthe elevated temperature and pressure. From 1B, it shows that even asmall difference between the resonant frequencies of two detectors canresult in a substantial mismatch in the phase responses of the detectorswithin the resonant-response region.

TOOL-MODE NOISE AND ROAD NOISE

Noise in this specification is energy travelling within or on thesurface of the logging tool that is not representative of the earth -borehole response to the transmitted signals. The term "noise" when usedin this specification shall include tool-mode noise and/or road noise.Noise may interfere with the ability of the detector transducers toprovide an accurate representation of the earth - borehole response.This may occur when noise of sufficient amplitude is detected along withthe borehole waves. Noise can be produced by any elastic waves travelingon the surface of or within the body of the acoustic logging tool.

Road noise is a noise at low frequency (approximately in the range of0-1 kHz) due to the banging of the tool against the side of theborehole. Although most serious for transverse wave detection, it isalso a problem for Stoneley wave detection. The conventional method forreducing the effects of road-noise on the signal is summing many signalsusing the incoherence of the roadnoise. But summing or stacking "N"traces only reduces the noise by ##EQU1## and since the road-noise couldbe hundreds of times larger than the signal of interest, it is stilloften a problem.

Tool-modes are various waves (modes) propagating along or within thelogging tool. These waves are referred to herein as "tool-mode" noise.This noise often occurs at frequencies close to the borehole signals andthe noise also propagates at velocities close to the velocities of theborehole signals. Tool-mode noise is coherent and cannot be suppressedby stacking.

If tool-mode noise and road-noise could be decoupled within the receiverarray so that it does not propagate throughout the array to couple toall of the transducer detectors in the array and if the acoustic signalscould be detected so that they are not subject to amplitude andfrequency distortion between the input signal and the output responsesignal, it would be possible to obtain composite signals which wouldaccurately reflect selected borehole propagation modes. The compositesignals from different receiver stations could then be used for anaccurate determination of the changes in phase and amplitude of theborehole response over time and distance, from which formationproperties can be inferred.

In addition, borehole waves are subject to the effect of resonances. Theborehole resonant frequencies are dependent in part upon the mode ofpropagation of the transmitted signal as well as borehole geometry. Theuse of a multi-mode receiver sonde having a flat frequency responseregion over a selected bandwidth which includes significantly all of theborehole resonant-response regions of interest would provide in a singlereceiver sonde the ability to accurately detect the amplitude and phaseof the acoustic signals across any borehole resonant-response regionthat is excited by any transmission mode of interest.

SUMMARY OF THE INVENTION

An object of this invention is to provide an acoustic logging receiverarray adapted to significantly prohibit (i.e., decouple) any noiseacross the receiver array, thereby reducing noise at any transducerdetector in the array.

Another object of this invention is to provide an acoustic loggingreceiver array which provides output response signals from the receiverstations which have substantially the same amplitude and phase responseto a common input signal, thereby allowing faithful recording ofborehole acoustic signals.

In accordance with some objects of the present invention, there isprovided a receiver sonde adapted to decouple noise (tool-mode noise androad-noise) within a semi-rigid receiver array so that the noise doesnot propagate across any portion of the array, thus, preventing (orsubstantially reducing) the tool-mode noise from coupling to transducersin the array. In one particular representative embodiment, thesemi-rigid receiver array includes: a top and bottom bulkhead andsemi-rigid receiver array-elements. The semi-rigid receiverarray-elements include rigid and massive receiver stations (receivingchassis-elements) to which detector transducers are attached, andcompliant, light spacers. The compliant spacers are positioned withinthe receiver array to separate the receiver stations from each other andfrom the top and bottom bulkhead- The compliant spacers also act tomaintain the receiver stations in their approximately fixed positionwithin the receiver array.

Noise cannot propagate across any portion of the receiver array for atleast two reasons. Firstly, the compliant elements act as vibrationisolators, which absorb most of the energy propagating across them.Secondly, because of the large disparity in the elastic properties ofthe compliant spacers and the receiver chassis elements, most energy notabsorbed across the compliant spacers is scattered at the junctionbetween the compliant spacers and the receiver chassis elements andfurther absorbed within the body of the receiver array. As such, theentire receiver array acts as a lumped vibration isolator which onlyallows very low frequency tool-noise (outside the bandwidth of interest)to propagate across the receiver array.

In addition, other embodiments are presented which provide for wide-banddetection of acoustic signals within the borehole. Wide-band detectionin this specification refers to the detecting of an acoustic signalwithin a selected bandwidth which has an approximately flat frequencyresponse for the receiver transducer, i.e constant ratio in amplitudeand substantially little phase difference between the input signal andthe output signal of the detector. Consequently, the output signal is afaithful representation of the input signal. Thus, an accuratedetermination of the acoustic signals in the borehole and of the changesin phase and amplitude between acoustic signals detected by separatedreceiver stations is obtained by the wide-band detection of acousticsignals. Wide-band detection is achieved by selecting transducerdetectors which have approximately the same frequency responsecharacteristics to an input signal and whose lowest resonant-responseregion is above the highest frequency of the selected bandwidth for thedetected signals.

Other embodiments are also provided, such as a receiver array which hasa common electrical ground for all output response signals from thetransducer detectors on a receiver station or for all of the outputresponse signals in a receiver array. Another embodiment is a receiversonde which has a multi-mode capability for detecting acoustic signalsby combining individual signals into composite signals which arerepresentative of various borehole-propagation modes.

These and other objects and advantages of the present invention will nodoubt become apparent to those of skill in the art after having read thefollowing detailed description of the preferred embodiments which arecontained herein and illustrated by various figures.

The invention encompasses the heretofore described preferred embodimentsas well as other embodiments as are described hereinafter and as will beapparent to those of skill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a prior art representation of a transducer amplitude vs.frequency response characteristic curve.

FIG. 1B is a prior art representation of a transducer phase vs.frequency response characteristic curve featuring the samerepresentative transducer as FIG. 1A.

FIG. 2 is a simplified perspective illustration of a well logging toolin a borehole with partial cutaway view of one embodiment of asemi-rigid receiver array of the present invention.

FIG. 3A is a simplified perspective illustration of an experimentalreceiver array having eight receiver stations.

FIG. 3B is a representation of the signals received by the transducerdetectors located on the experimental receiver array of FIG. 3A.

FIG. 4 is a more detailed side cross sectional view of the embodiment ofthe semi-rigid receiver array of FIG. 2 taken at section 4--4.

FIG. 5 is a cross sectional bottom view of the embodiment of thereceiver station of FIG. 4 taken at section 5--5.

FIG. 6 is a perspective side view of one embodiment of a receiverstation based upon one embodiment of the receiver station shown in FIG.5.

FIG. 7 is a simplified electrical block diagram of one embodiment of amulti-mode receiver station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is an acoustic logging receiver apparatus adapted todecouple noise within one portion of a receiver array from detectortransducers located in the other portions of the receiver array. Theinvention may also include wide-band signal detection which in thisspecification means the detection of signals which have substantiallylittle amplitude and phase distortion between an output response signalfrom the detector and an input signal to the detector over a selectedbandwidth. The apparatus may also include multi-mode detection fordifferent borehole-propagation modes (monopole, dipole, quadrupole,octopole, etc.). The apparatus may be mounted, preferably, in a separatesonde from the transmitters, or in combination with one or more acoustictransmitters mounted within the same sonde. In addition, this receiverapparatus may be used in conjunction with acoustic signals generated inor outside of the borehole to detect signals.

FIG. 2 is a simplified perspective view of a well logging tool accordingto one particular embodiment of the present invention.

Well logging tool 10 is shown suspended from communication cable 12 in aborehole 14 containing a liquid. In this embodiment, a transmitter sonde16 is shown located above a receiver sonde 18. Between these sondes is atube spacer 20. Attached to the outside surface and around transmittersonde 16 and receiver sonde 18 are flexible rubber "finger" centralizers22 which act to maintain the sondes in the center of the borehole 14.

The exterior of the transmitter sonde 16 is typically an elongatedmetallic tube with openings for acoustic windows (not shown in FIG. 2)to allow acoustic signals to be transmitted from the interior of thetransmitter sonde into the fluid of the borehole.

The transmitter sonde 16 contains one or more acoustic transmitterscapable of transmitting compressional waves into the borehole fluid inat least one or more of the following transmission modes: monopole,dipole, quadrupole, octopole or other forms of multipole transmission.In addition, the transmitter sonde 16 may contain additional componentsused for the collection of downhole data such as a gamma ray detectorand/or 10 instrumentation for sending data to the surface or forreceiving control signals from the surface. The use of the descriptiveterm "transmitter sonde" is not meant in this specification to limit thetransmitter sonde 16 to containing only acoustic transmitters.

The receiver sonde 18 of this embodiment contains a semi-rigid receiverarray 24 (shown in a simplified perspective cut away view and in apartial cross section) which is surrounded by an exterior jacket 26(shown in cross section). The exterior jacket 26 of this particularembodiment is a tubular metallic shield with acoustic windows 27. Thereceiver sonde 18 also includes a receiver electronics section 28.Attached to the receiver sonde 18 of this particular embodiment is asinker bar or hole finder 30.

The receiver electronics section 28 contains a remotely controlledelectronics package that includes a gain control. The electronicspackage of this embodiment also contains a composite signal means forcombining signals detected within the receiver array 24. The electronicssection 28 also suitably contains an electronics package which sendssignals to the surface by way of the armored communication cable 12. Thecommunication cable 12 is connected to the receiver electronics section28 by internal wiring. The receiver electronics section 28 also suitablycontains additional components used for collecting downhole data such asa magnetometer and/or other electronic components. The use of thedescriptive term "electronics section" is not meant in thisspecification to limit this section to containing only electroniccomponents related to acoustic signal acquisition.

The semi-rigid receiver array 24 of this particular embodiment, includestwo receiver stations (receiving chassis-elements) 32 having detectortransducers 33 attached thereon and weights (non-receivingchassis-elements) 34 positioned above and below the receiver stations 32and in proximity to a top bulkhead 36 and a bottom bulkhead 38. The topbulkhead 36 and bottom bulkhead 38 are mechanically and electricallycoupled to exterior bulkheads 40. The exterior bulkheads help tomaintain a liquid tight seal on the semi-rigid receiver array and alsoallow for mechanical and electrical coupling to the rest of the welllogging tool. An inner jacket 42 (shown in cross section in FIG. 2)provides the liquid seal for the longitudinal periphery of thesemi-rigid receiver array by acting as a barrier between the liquid ofthe borehole and the interior of the receiver array. In this embodiment,the inner jacket 42 is an elastomeric sleeve. The interior of thereceiver array 24 is filled with an insulating fluid 44.

In other embodiments, the inner jacket may be used without an exteriorjacket 26, or the inner jacket and the outer jacket are suitablycombined such that there is an elongated rigid tube with elastomericacoustic windows to allow the acoustic signal to enter the receiverarray to be detected- In such a case, the term "inner jacket" will alsoapply to this type of configuration in this specification.

The receiver stations 32 (receiving chassis-elements) and weights 34(non-receiving chassis-elements), in the embodiment of FIG. 2, areseparated and maintained in an approximate fixed position by thecompliant spacers 46 with respect to: each other, the top and bottombulkhead, and their longitudinal location within the receiver array 24.In this particular embodiment, the compliant spacers are tubular inshape and are formed of an elastomeric material. The compliant(semi-rigid) spacers fix the position of receiver stations 32 inproximity to the acoustic windows 27 in the outer jacket 26.

NOISE WITHIN A RECEIVER ARRAY

In some conventional acoustic logging tools, the transmitter sonde 16and the receiver sonde 18 may be coupled together into a singlecontinuous tube. However, with a single sonde configuration, the signalsgenerated by the transmitters for transmission into the borehole mayalso propagate within the body or on the surface of logging tool. Ifthese signals within or on the surface of the logging tool are picked upby the detectors, these signals could interfere with the ability of thelogging tool to obtain from the borehole usable signals which containthe borehole or earth response to the transmitted signals. Variousmethods have been developed to reduce the impact of the coupling of thetransmitter signals which propagate within the body of the tool to theoutput response signal from the detectors.

One such conventional method is the separation of the transmitter sondefrom the receiver sonde by using the tube spacer 20 shown in FIG. 2.However, other forms of the noise may affect the receiver sonde, such asborehole waves coupling to the surface of or interior components of thereceiver sonde or road noise coupling to the receiver sonde due to thescraping and banging of the receiver sonde against the side of theborehole. Any of these noises may be detected by the transducerdetectors. The output response signals from the detectors would theninclude both the acoustic signal from the borehole and the noise fromthe logging tool.

One major path for tool-mode noise and/or road-noise to propagate in thereceiver array and to be picked up by the transducer detector is via thechassis upon which the detector is mounted in the receiver array.

A conventional receiver array in prior art has a single rigid chassismade of one continuous (or solid) component which will be referred toherein as an "array-chassis" that may be rigidly coupled to the rest ofthe receiver sonde. The array-chassis supports a plurality of receiverstations longitudinally within the array. The prior art arrangement forsupporting receiver stations on a rigid continuous array-chassis allowsthe noise to move within or on the surface of the array-chassis tocouple to all of the detector transducers. The noise may enter thearray-chassis through connections between the array and the body of thereceiver sonde or through the insulating fluid to various portions ofthe array. There is no specifically included active mechanism withinthis rigid solid array to prevent frequency components of the noise nearthe resonant frequencies of the array-chassis from resonating within thearray-chassis. These resonating components of the noise could havesignificant amplitudes. In addition, there are no specifically includedactive mechanisms within this array-chassis to reduce or preventnon-harmonic frequency components of the noise, i.e., frequencycomponents which are not near the resonant frequencies of thearray-chassis. Thus, the array-chassis provides an ideal propagationpath for the noise which enters the array from any location to couplevia the array-chassis to any or all of the transducer detectors whichare positioned throughout the receiver array.

DECOUPLING OF NOISE WITHIN THE RECEIVER ARRAY

This invention decouples noise in one portion of the receiver array fromthe transducer detectors located in another portion of the receiverarray. The term "decouple" in this specification means that energy whichenters the receiver array at any point in the array cannot propagate (orat least is significantly reduced in amplitude) to other portions of thearray. The net effect is that energy can only enter the receiver arrayat a transducer detector via the borehole fluid, thereby allowing for atrue reconstruction (representation) of the acoustic field in theborehole.

Returning to FIG. 2, the decoupling of the noise is accomplished byusing compliant spacers 46 between massive receiver elements. Acompliant spacer is made up of viscoelastic material which absorbsvibrational energy travelling across it, i.e. it acts as a vibrationalisolator. The receiver chassis elements 32 are separated from each otherand from the bulk-heads by compliant spacers 46. In this embodiment, thecompliant spacers are annular rubber sleeves, which are rigid enough tosustain their shapes and the linear shape of the receiver array. Asvibrational energy travels across the receiver array from any point inthe array, it is absorbed partially by the compliant spacers. Any energynot absorbed is scattered at the junction between the compliant spacers46 and the receiving elements 32, because of the large disparity in theelastic properties of the compliant spacer and the receiving chassiselement (impedance mismatch). The scattered energy is further absorbedby the compliant spacer and also displaced within the body of the tool.It is evident that the absorption of energy will be cumulative with thenumber of compliant spacers and compliant spacer-receiver-chassisjunctions. As such, the entire receiver array may be viewed as a set ofoverlapping vibrational isolators. The receiver stations closest to thebulkheads 36 and 38 are particularly susceptible to large road noise orother tool-mode noises which will enter the receiver array at thebulkheads. Whereas introducing a compliant spacer between the bulkhead36 and 38 and the nearest receiving chassis-element 32 is usuallysufficient, in the embodiment of FIG. 2, a non-receiving chassis element34 is inserted with another compliant spacer 46 between thenon-receiving chassis-element 34 and the receiving chassis-element 32.The insertion of the non-receiving chassis-element 34, or weight, allowsthe introduction of extra compliant spacers for extra absorption oftool-mode energy before it reaches a detector transducer.

The ability of this invention to reduce noise within a semi-rigidreceiver array is shown in FIGS. 3A and 3B. FIG. 3A shows a sideperspective view of an experimental configuration for a receiver array.This experimental receiver array has eight receiver stations identifiedas 32a through 32h. Stations 32a through 32h are separated by semi-rigidcompliant spacers 46. Station 32a and 32h are also connected directly tothe upper and lower bulkheads 36, 38 by rigid annular steel chassiselements 47. FIG. 3B is a representation of the signals received by thedetector transducers on receiver stations 32a through 32h of thisinvention when subjected to a noise. Receiver stations 32a and 32h didnot have the benefit of a compliant spacer between the receiver stationand the bulkheads 36, 38. It can be seen that noise 48 almost completelyoverrides two forms of acoustic waves from the borehole, a compressionalarrival 49a and a flexural mode arrival 49b at receiver stations 32a and32h. Whereas, receiver stations 32b through 32g which are separated bycompliant spacers and have the benefit of vibrational isolation, providecompressional arrival 49a and flexural mode arrival 49b signals whichare not distorted or overridden by the noise 48.

The signals obtained at stations 32b and 32g indicate that some (highlyattenuated) noise has been picked up by the transducer detectors ofthese receiver stations. Whereas, the signals obtained by 32c through32f have less and in addition, the noise on the signals appears to beabout equal. The experimental configuration of FIG. 3B does not ensurethat the array provides for equal amounts of decoupling between thereceiver stations.

The embodiment of FIG. 2 resolves this problem by using the weights (thenon-receiving chassis-elements) 34 to ensure that an equal amount ofdecoupling is provided at each receiver station (receivingchassis-element) 32. The weights allow for extra compliant spacers andextra scattering centers to be interspersed along the array,particularly between the lower and upper bulkheads and the center of thearray.

In other embodiments of this invention, when substantial decoupling ofthe noise propagating from the bulkheads is obtainable without the useof the weights 34, the non-receiving chassis elements may not be used;or, in other embodiments it may not be necessary to ensure that theamount of decoupling within the receiver array is balanced at eachreceiver station.

In embodiments of this invention, the number of receiver stationsincluded within each receiver array is determined by the number ofsignals or the number of composite signals desired from each semi-rigidreceiver array.

RECEIVER ARRAY

Preferred embodiments of this invention, as mentioned previously, arecapable of decoupling noise within the receiver array and of wide-banddetection of an acoustic signal.

FIG. 4 illustrates a more detailed partial side cross sectional view ofthe semi-rigid receiver array embodiment (the inner jacket is not shown)of FIG. 2 taken at section 4--4. This embodiment is configured forwide-band multi-mode detection of an acoustic signal. However, certainfeatures related to this embodiment are also beneficial for conventionaldownhole acoustic detection techniques and these features should be alsobe considered within the scope of the claims of this invention.

FIG. 4 illustrates the components of one embodiment of a semi-rigidreceiver array which decouples noise in one portion of the receiverarray from other portions of the receiver array. This embodimentindicates how the detectors may be wired to obtain a common electricalground for the detector transducers attached to the receiver stationthereby providing for a reduction in the wiring necessary tointerconnect the receiver array with the receiver electronics package.

FIG. 4 shows the semi-rigid receiver array 24. The semi-rigid receiverarray includes chassis-elements 32, 34 and compliant spacers 46. Theembodiment of FIG. 4 has two receiver stations 32. Positioned above andbelow the receiver stations (receiving chassis-elements) 32 are weights(non-receiving chassis-elements) 34. The weights are located, aspreviously described, in proximity to the top bulkhead 36 and the bottombulkhead 38. The receiver stations 32 and weights 34 are separated andmaintained in an approximate fixed position with respect to each other,the top and bottom bulkheads, and their longitudinal location within thereceiver array 32 by the compliant spacers 46. The compliant spacers 46are preferably, as previously described, made of an elastomericviscoelastic material providing a dissipation mechanism to noise. Inthis particular embodiment, the compliant spacers are annular rubbersleeves and, in preferred embodiments, the compliant spacers separatethe receivers stations to have approximately the same distance betweenthe transducers on separated receiver stations.

The compliant spacers 46 are held in position to separate the weights34, the top and bottom bulkheads 36, 38 and the receiver stations 32 bycompression bands 50.

However, other methods known in the art may be used to maintain thesemi-rigid spacers in a position between and in contact (orcommunication) with the chassis-elements 32, 34 and/or the top andbottom bulkheads 36,38, such as, using screws, clamps or pressurefittings.

In this particular embodiment, a stress cable 52 (although not anecessary component in other embodiments) runs through an opening 54 inthe center of the receiver stations 32 and the weights 34. The stresscable 52 is used to support the weight of the receiver array 24 andother components which may be connected to the logging tool below thereceiver array 24. In this embodiment, compliant spacers 46 arepreferably only rigid enough to maintain the spacing and to prevent thereceiver array-elements from contacting the stress cable 52 (the stresscable is a conduction path for noise) when the receiver array 24 isoperated in its normal approximately vertical position.

Communication wiring 56 is shown entering the receiver array 24 at thetop bulkhead 36 through an opening which is made possible by securingstress cable 52 to the top bulkhead by an eyebolt connection 58. Thecommunications cable 56 passes through the center of the receiver array24 and joins other wiring such as receiver detector wiring 60. Thewiring then passes through the bottom bulkhead 38 by way of notches 62cut in the lower most part of the bottom bulkhead 38 to connect to thereceiver array electronic packages which in this particular embodimentare located in another part of the receiver sonde.

The openings shown at the top and bottom bulkheads 36, 38 and the mannerof connecting the stress cable 52 are for illustrative purposes only andmay be done in any manner known in this art for fabricating bulkheadsfor acoustic downhole tools and connecting the various individual partsor components (mechanical and electrical) together to form a welllogging tool.

RECEIVER STATION CHASSIS/COMMON ELECTRICAL GROUND

Each receiver station 32 includes a chassis 64, upon which are mountedtransducers, preferably pressure-sensitive piezoelectric transducers, 66to act as transducer detectors. In this particular embodiment, thechassis 64 are formed of an electrically conductive metal. The mountingof the transducers 66 upon the chassis 64 is done in a manner such thatone surface of the transducer 66 is in contact with the chassis 64 andelectrically continuous with the chassis 64. In this embodiment, thetransducer 66 has a conductive electrode plated on the inner face, i.e.,the transducer face adjacent to the chassis 64. The inner face of thetransducer 66 is bonded to the chassis. Bonding of the inner face of thetransducer 66 to the chassis in this particular embodiment provides fora common electrical ground for the output signals of all the detectorsmounted in this manner upon a single chassis 64.

Other methods may be used to establish a common electrical ground pathin similar embodiments of this receiver array including the use ofnonconductive material for the chassis 64 with wiring or conductiveplating used to establish a common ground for each detector.

The two chassis 64 of FIG. 4 are shown to be connected electrically by acommon ground wire 68 which provides for a common electrical groundbetween all receiver stations 32 in this embodiment. An instrumentground wire 70 is shown connected to one of the chassis (the lowerchassis in this drawing). This wire leaves the receiver array 24 throughthe bottom bulkhead 38 of this particular embodiment to connect to thereceiver electronics package to provide the instrument ground path forthe detectors of this receiver array.

In this embodiment, the outer face of each transducer 66 is plated witha conductive electrode. Wiring paths 72 are made through chassis 64 toprovide one method to connect detector wiring 60 to the outer face ofeach transducer 66. Although other methods are known in the art, in thisembodiment, the detector wiring 60 is held in place by insulators 74within the wire path 72, the insulation on the wiring is stripped off ofthe detector wiring 60 and then connected to the conductive electrode onthe outer face of the transducer 66. The opposite end of the detectorwiring 60 is connected to the receiver electronics package located belowthe receiver array.

Referring now to FIG. 5, a cross sectional bottom view of the receiverstation 32 depicted in FIG. 4 at section 5--5 is provided. In thisembodiment, the chassis 64 has a circular periphery (other shapes aresuitable) with the opening 54 through the center of the chassis forwiring 75 (for example, communication and detector wiring) and thestress cable 52 to pass through. Transducers 66 are shown on oppositesides of the chassis 64. In this embodiment, a detector cavity 78 is cutinto the periphery of chassis 64 to provide a flat surface for the flatbottom surface of the transducers 66 to be electrically and mechanicallybonded to the chassis 64. As is apparent to those skilled in the art,other piezoelectric detectors having a different poling axis or shapesmay be used for other embodiments. As one example, a piezoelectricdetector could be used which has a radial shape with a thickness polingaxis. In this case, the bonding surface on the chassis may be shaped tocorrespond to the shape of the bottom surface of the transducer used forthe detector.

CHASSIS TRANSDUCER PLACEMENT FOR MULTI-MODE DETECTION

Four transducers 66 are shown in FIG. 5 for the receiver station. Thesignals from these four detectors may be combined electronically to forma composite signal (to be described in more detail hereinafter) so thatthis embodiment of the invention may detect in any of the followingmodes: monopole, dipole (two dipole modes available, orthogonal to eachother) and quadrupole. It is possible to place six, eight, or moretransducers in paired combinations on a single chassis to enable areceiver station to detect additional borehole-propagation modes. Apaired combination is two transducers with one transducer having acorresponding transducer on the opposite side of the chassis. Eachpaired combination is separated from the other paired combinations andother individual detectors by equal spacing around the periphery of thechassis. For example, FIG. 5 shows two paired combinations of detectorswith axial locations on the periphery with respect to the center of thechassis of 90 degrees of angular separation or spacing between eachtransducer detector. The alignment of paired combinations of detectorswith respect to the center of a borehole to obtain differentborehole-propagation modes with detectors is well known in the art.

Referring now to FIG. 6, a perspective side view of the receiver station32 of the embodiment of FIG. 5 is shown. In the cavity 78 on chassis 64is a pressure-sensitive transducer 66. Detector wiring 60 is shownconnected to the outer face of transducer 66 and insulator 74 is shownisolating the detector wiring 60 from chassis 64. In this embodiment,transducer 66 is a rectangular cut pressure-sensitive piezoelectricdetector poled in the thickness direction. This mode of piezoelectrictransducer operation is referred to as the TE (thickness expansion)mode.

The TE mode is preferred for this embodiment. The thickness of thepiezoelectric detector, in general, determines the resonant frequency ofthis mode of operation. By using this particular mode, the size of thedetectors may be reduced so that the detectors approach a point source,with respect to the distance between separate receiver stations, for thedetection of acoustic signals from the borehole. This provides a moreaccurate indication of spatial resolution between the same signalsdetected on different receiver stations. In some prior art conventionaldetection modes which did not use the TE mode, the detectors may havebeen selected because their resonant-response region overlaps theparticular borehole resonance-response region where signals are to bedetected. Conventional detectors selected in this manner could approach5 inches in length so that the resonance-response region of thedetectors could overlap the borehole resonance-response region. If thereceiver stations had separation distances of 6 inches, it was notpossible in this conventional form of detection to accurately determineseparation distances between the signals detected on the separatereceiver stations because the acoustic signals could have been detectedat any location along the length of each detector.

Since a TE mode detector can be made smaller without affecting theresonant-response characteristics of the detector, more detectors may beplaced around the periphery of the receiver station. Thus, when theacoustic tool uses smaller detectors, it may detect a larger number ofborehole-propagation detection modes.

However, in general, the smaller the size of the detector, the smallerthe output response signal. Consequently, the output response signals ofan embodiment having small detectors, in particularly the TE mode, mayrequire substantially more amplification than conventional largerdetectors.

Additionally, in this embodiment, as well as similar embodiments, it ispreferred that the poling axis of each detector be mounted in the samepoling direction with respect to the center of the chassis, i.e. allfacing radially outward or all radially inward, so that a common groundsystem for different borehole-propagation modes is established byproviding a common polarity for the output response signals on eachreceiver station.

It is also possible, as indicated previously, to use other shapes ofdetectors for other embodiments of this invention than the rectangularshape shown for the thickness poled detector transducers of thisembodiment such as a square or circular shape.

This invention also contemplates the use of piezoelectric detectorswhich are formed of a plurality of single piezoelectric elements joinedto form a single detector element, such as, for example, but not limitedto, a bender bar transducer. One possible arrangement for a bender bartransducer would be to attach the bender bar to the sides of a cavitysuch as the cavity 78 shown in FIG. 6 so that the detecting bender barsurfaces (the outer face and inner face) are not restricted in movement.A common ground wire connection may, as an example, be made to the lowersurface of each bender bar.

WIDE-BAND TRANSDUCER SELECTION CRITERIA

In one preferred embodiment for the wide-band detection of acousticsignals of this invention, it is recommended that the resonant frequencyof the transducer detector, and in particular for a piezoelectricdetector, be on the order of ten times or more than the maximumfrequency within the selected bandwidth. For example, returning to FIGS.1A and 1B, using a selected bandwidth of 200 to 2,000 Hz in the signalband designated as 79, the transducer having this frequency outputresponse curve would not meet the before mentioned criteria because ithas a resonant frequency of approximately 10 kHz with a departure fromthe flat frequency response characteristics at approximately 3 kHz. Eventhough the response of the selected bandwidth 79 is within theapproximately flat portion of the frequency response curves on FIGS. 1Aand 1B, this transducer would not be acceptable in this particularpreferred embodiment.

The transducer that is selected to meet the criteria of this example hasa resonant frequency of at least 20 kHz. All transducer detectors forthe same receiver station and preferably all of the transducer detectorswithin a receiver array where the composite signals are to be comparedreceiver station to receiver station should have approximately the samefrequency response characteristics to an input signal including havingapproximately the same lowest resonant frequency. This ensures that thesignals in the selected operating bandwidth detected by each detectorhave the same amplitude for a common input signal and that the signalsremain in the approximately flat frequency response region even if thetransducers are subject to significant resonant frequency drift underthe varying temperatures and pressures of the borehole. Thus, thecomposite signal of the receiver station is made up of output responsesignals which have similar frequency responses in amplitude and phase.

An accurate indication of the amplitude and phase of the detectedborehole propagation-mode is even more significant when several receiverstations are present in a receiver array and the composite signals fromthe receiver stations are to be compared during processing. With theselected bandwidth of all the detectors maintained in the approximatelyflat frequency response region of the frequency response curves ofsimilar detectors, the received signals (or composite signals) withinthe selected bandwidth will still have the same relationship to eachother, i.e., approximately the same frequency response characteristicsin amplitude and phase, from receiver station to receiver station in theevent the individual resonant frequencies of the detectors drift; thus,accurate determinations of phase and amplitude differences between thecomposite signals over distance and time are obtained when the compositesignals are compared.

However, for wide-band detection, it is not always necessary to ensurethat the resonance frequencies of the detectors are 10 times greaterthan the highest frequency in the selected bandwidth. As long as thedetected signals are obtained from similar detectors and the signalswithin the selected operating bandwidth are below the lowestresonant-response region of the detectors when the signals are detected,wide-band detection is achieved; the detected signals have substantiallylittle amplitude and phase distortion between the input signal and theoutput response signals.

MULTI-MODE DETECTION

From the transducer detectors, the output response signals pass into thereceiver electronics section. FIG. 7 is a simplified electrical blockdiagram of one embodiment of how this invention may operate as amulti-mode receiver station.

In FIG. 7, piezoelectric detector elements 66a, 66b, 66c and 66d arerepresented to have the same axial configuration as shown by thedetectors 66 in FIG. 5, however, the suffixes have been added to showhow the detector elements at different axial locations are combined toobtain composite signals.

In this particular embodiment, wire 60 (an ungrounded output wire)connects to the outward face of each transducer detector 66a, 66b, 66cand 66d. Each detector is also shown to have a common electrical(ground) potential which is connected to instrument ground wire 70. Inone embodiment, the output wire 60 of each detector could be feddirectly into a remotely controlled electronics package 82. However, inthis preferred embodiment the output of each individual detector is fedto a charge amplifier 80 which amplifies the output response signal andenhances the ability of the receiver electronics to handle lowfrequencies from the detector.

In this embodiment, a band-pass filter 82 is applied to individualsignals before the signals enter the remotely controllable electronicspackage 84 to remove detector frequency responses outside the selectedbandwidth. In other embodiments, a filter may be applied after acomposite signal is formed in the electronics package 84 or after thesignals have been transmitted to the surface. In addition, otherprocessing techniques may be applied where a band-pass filter is notnecessary or desired.

The filtered individual signals then enter the remotely controlledelectronics package 84. The electronics package 84 contains a compositesignal means (remotely controllable from the surface) for combining theoutput response signals from the detectors to provide a composite signalwhich is representative of a borehole-propagation mode for the detectedacoustic signal. The composite signal is formed by combining an outputresponse signal from at least one of the detector transducers with atleast one output response signal from the remaining detectortransducers. In addition, the composite signal may include only oneoutput response signal from the detectors when a single detectormonopole response is desired.

In this preferred embodiment, the remotely controlled composite signalmeans provides electronic switching of the outputs from the detectors toform a composite signal using summing/differencing amplifiers. Thecomposite signal is representative of the remotely requestedborehole-propagation detection mode. Consequently, the receiver sonde ofthis embodiment is capable of remote multi-mode operation.

In this embodiment, the composite signal is then sent up-hole by anelectronics package 86 within the receiver section to a surfacerecording system. The composite signal is then processed by variousmethods to determine information about the formations which the acoustictool passed through.

From the physical configuration of the four detectors shown in FIG. 7and in consideration of the common signal polarity of the outputresponse signal from the detectors of this particular embodiment, themulti-mode receiving apparatus may obtain the following composite signalmodes as an output from the remotely controlled composite signal meansof the electronics package 82: (1) the detection of any one of thesignals from detector elements 66a, 66b, 66c and 66d, (2) the summationof all of the signals will provide a monopole composite signal, (3)subtracting (that is, reversing the polarity) the signal of 66a from thesignal of 66c, or subtracting the signal of 66b from the signal of 66dwill provide a dipole response, and (4) subtracting the signal of 66afrom the signal of 66b and adding that response to the signal obtainedby subtracting the signal of 66c from the signal of 66d will provide aquadrupole response. Similar combinations of signals could be made inother embodiments which feature additional detectors combination pairsto provide composite signals for other borehole-propagation detectionmodes.

In another embodiment, the receiver electronics section may not processthe individual signals through the composite signal means; but instead,the electronics section provides for each individual output responsesignal to be transmitted to the surface so that through processing ofthe individual signals, many different types of composite signals areobtainable from a group of output response signals. In this way, manyborehole-propagation detection modes may be analyzed. Additionally, theremotely controlled composite signal means of the electronics package 84may be programmed in other embodiments so that both composite dipolesignals, i.e., dipole signals orthogonal to each other from the samereceiver station, are transmitted to the surface. 15 In anotherembodiment, instead of a remotely controlled composite signal means, thecomposite signal means may be selected by physically arranging thewiring of the composite signal means to a specific detection mode at thesurface before starting a logging run. 20 While several embodiments ofthe invention have been shown and described, it will be understood thatthe invention is not limited thereto since many modifications may bemade and will become apparent to those skilled in the art.

What is claimed is:
 1. A method for reducing the effects of noise oncomposite signals obtained with a receiver sonde in a liquid containingborehole, the noise being present in the borehole and including at leastone of two types of said noise, the two types of noise being aroad-noise and a tool-mode noise, the method comprising the steps of:(a)attaching transducers to chassis to form receiver stations; (b)separating the receiver stations from each other with compliant spacers,the compliant spacers being in contact with the chassis and forming ajunction having a disparity in elastic properties where the compliantspacers and chassis are in contact; (c) disposing the receiver stationsand the compliant spacers longitudinally within the sonde to form asemi-rigid receiver array; (d) separating the semi-rigid receiver arrayfrom the rest of the sonde with said compliant spacers; (e) locating thesonde within the liquid containing borehole; (f) generating acousticwaves within the borehole, the acoustic waves providing the inputacoustic signal to each transducer of said transducers; (g) decouplingthe noise in the borehole as it propagates along the semi-rigid receiverarray by absorbing the noise as it propagates across each compliantspacer and scattering the noise at it attempts to propagate across eachsaid junction having a disparity in elastic properties; (h) producing anoutput response signal from each said transducer from the input acousticsignal, the output response signal having a reduced amount of noise onthe response signal as a result of the decoupling of the noise along thesemi-rigid receiver array; and (i) selectively combining output responsesignals from each receiver station to form a composite signal for eachsaid receiver station.
 2. The method for reducing the effects of noiseon composite signals in accordance with claim 1, wherein:step (c) alsoincludes, positioning a non-receiving chassis at each end of thesemi-rigid receiver array, each non-receiving chassis being separatedfrom an adjacent said receiver station by a compliant spacer, each saidcompliant spacer being in contact with the non-receiving chassis andforming another junction having a disparity in elastic properties, thissaid junction being formed where the non-receiving chassis and saidcompliant spacer are in contact; and step (d) includes separating thenon-receiving chassis element located at each end of the semi-rigidreceiver array from the rest of the sonde with said compliant spacerbetween each said non-receiving chassis element and the rest of thesonde.
 3. The method for reducing the effects of noise on compositesignals in accordance with claim 1, wherein:step (a) also includes,selecting transducers poled in the thickness expansion mode andattaching each transducer to the chassis such that each said transduceris poled in the same radial direction with respect to the chassis. 4.The method for reducing the effects of noise on composite signals inaccordance with claim 3, wherein:step (a) also includes, grounding thetransducers to have a common electrical ground by selecting chassiswhich have a conductive path and attaching the transducers to thechassis such that the transducers are connected to the conductive pathto establish the common electrical ground on each said chassis, whereby,the output response signals of each said receiver station have a commonpolarity with respect to each other; and step (a) additionally includes,grounding the receiver stations together to establish a commonelectrical ground by connecting the common electrical ground on eachsaid receiver station together, whereby, the composite signal of eachsaid receiver has a common polarity with respect to other said compositesignals.
 5. The method for reducing noise on composite signals accordingto claim 4, wherein:step (a) also includes, providing multi-modereceiver stations by attaching the transducers to each said chassis inpaired combinations to provide a multi-mode receiver station, a pairedcombination being two transducers positioned on opposite sides of thechassis with each said transducer of the paired combination beingequally spaced axially around the periphery of the chassis with respectto the other said transducers; and step (h) also includes, detecting aselected bore-hole propagation mode by selectively combining the outputresponse signals from each said receiver station such that the compositesignal is representative of the selected borehole-propagation mode. 6.The method for reducing the effects of noise on composite signalsaccording to claim 5, wherein:step (a) also includes, selectingtransducers which are pressure sensitive, flat, piezoelectrictransducers; and step (a) additionally includes, establishing the commonground on each said chassis by selecting the chassis to have aconductive metal for the conductive path, and attaching thepiezoelectric transducers to the chassis by plating an inner face ofeach said transducer with a conductive electrode and bonding the innerface to the conductive metal of the chassis.
 7. The method for reducingthe effects of noise on composite signals according to claim 6,wherein:step (b) also includes, separating the receiver station withcompliant spacers having approximately the same length, whereby thereceiver stations are longitudinally separated from each other byapproximately the same distance; step (a) also includes, selectingpiezoelectric transducers which are small in comparison to the distancebetween the transducers on separated said receiver stations, whereby thesmall transducers approach a point source for detecting the inputacoustic signal; step (g) also includes, producing the output responsesignals to have an accurate indication of spatial resolution between theoutput response signals of the separate receiver stations; and step (h)also includes, providing spacial resolution between the compositesignals by combining output response signals having an accurateindication of spacial resolution between the output response signals ofthe separate receiver stations.
 8. The method for reducing the effectsof noise on composite signals according to claim 5, wherein:step (a)also includes, selecting an operating bandwidth for the compositesignals and selecting the transducers to have a wide-band response overthe selected operating bandwidth for the composite signals by choosingtransducers which have a lowest resonant-response region for eachselected transducer that is above the selected operating bandwidth, eachsaid selected transducer having approximately the same lowestresonant-response region, each said selected transducer also having aflat frequency response region over the selected operating bandwidth. 9.The method for reducing the effects of noise on composite signalsaccording to claim 8, wherein:step (a) also includes, selectingtransducers which have a lowest resonant frequency for each saidtransducer that is at least ten times greater than the highest frequencyin the selected operating bandwidth, each said transducer havingapproximately the same lowest resonant frequency.
 10. The method forreducing the effects of noise on composite signals according to claim 8,wherein:step (a) also includes, selecting transducers which are pressuresensitive, flat, piezoelectric transducers; and step (a) additionallyincludes, establishing the common ground on each said chassis byselecting the chassis to have a conductive metal for the conductivepath, and attaching the piezoelectric transducers to the chassis byplating an inner face of each said transducer with a conductiveelectrode and bonding the inner face to the conductive metal of thechassis.
 11. The method for reducing the effects of noise on compositesignals according to claim 10, wherein:step (b) also includes,separating the receiver station with compliant spacers havingapproximately the same length, whereby the receiver stations arelongitudinally separated from each other by approximately the samedistance; step (a) also includes, selecting piezoelectric transducerswhich are small in comparison to the distance between the transducers onseparated said receiver stations, whereby the small transducers approacha point source for detecting the input acoustic signal; step (g) alsoincludes, producing the output response signals to have an accurateindication of spatial resolution between the output response signals ofthe separate receiver stations; and step (h) also includes, providingspacial resolution between the composite signals by combining outputresponse signals having an accurate indication of spacial resolutionbetween the output response signals of the separate receiver stations.12. The method for reducing the effects of noise on composite signalsaccording to claim 10, wherein:step (a) also includes, selectingtransducers which have a lowest resonant frequency for each saidtransducer that is at least ten times greater than the highest frequencyin the selected operating bandwidth, each said transducer havingapproximately the same lowest resonant frequency.
 13. A receiver sondeadapted for reducing the effects of noise on composite signals that aredetected by the sonde in a liquid containing borehole, the noiseaffecting the composite signals in the borehole includes at least one oftwo types of noise, the two types of noise being a road-noise and atool-mode noise, the receiver sonde comprising:a top bulkhead and abottom bulkhead; a semi-rigid receiver array, the semi-rigid receiverarray including receiver stations and compliant spacers; each saidreceiver station having a chassis with transducers attached thereon,each transducer of the transducers being functional to produce an outputresponse signal from an input acoustic signal; the compliant spacers arein contact with and separate the chassis from the top bulkhead, thebottom bulkhead, and each other, the compliant spacers maintaining thechassis in an approximately fixed longitudinal position between the topbulkhead and the bottom bulkhead and enabling the receiver array to besemi-rigid, each compliant spacer of the compliant spacers beingfunctional to decouple the noise propagating within the semi-rigidreceiver array by acting as a vibration isolator to absorb the noise atit propagates across each said compliant spacer and by forming ajunction having a large disparity in elastic properties where each saidcompliant spacer contacts the chassis, each said junction scattering thenoise to reduce the amount of noise which propagates across thejunction, whereby, the amount of noise that reaches the transducers isreduced, thereby, reducing the amount of noise that is coupled to eachoutput response signal; and a composite signal means for combining theoutput response signal of each said transducer to provide a compositesignal for each said receiver station.
 14. The receiver sonde adaptedfor reducing the effects of noise on composite signals in accordancewith claim 13 wherein the compliant spacers are annular rubber sleeves.15. The receiver sonde adapted for reducing the effects of noise oncomposite signals in accordance with claim 13 wherein each saidtransducer is poled in the thickness expansion mode and each saidtransducer is attached to the chassis such that each said transducer ispoled in the same radial direction with respect to the center of thechassis.
 16. The receiver sonde adapted for reducing the effects ofnoise on composite signals in accordance with claim 15, alsocomprising:an electrical connection between the receiver stations;wherein each said transducer has an inner face with respect to thechassis; wherein each said chassis has a conductive path, the inner faceof each said transducer being connected to the conductive path toestablish a common electrical ground such that the output responsesignal from each said transducer attached to the chassis has a commonpolarity with respect to all output response signals of the chassis; andthe electrical connection between the receiver stations establishes acommon electrical ground such that each said composite signal has acommon polarity with respect to the other composite signals.
 17. Thereceiver sonde adapted for reducing the effects of noise on compositesignals in accordance with claim 16, also comprising:an electronicspackage; and wherein the transducers are attached to the chassis inpaired combinations to provide a multi-mode receiver station, a pairedcombination being two transducers positioned on opposite sides of thechassis with each said transducer of the paired combination beingequally spaced axially around the periphery of the chassis with respectto the other said transducers, whereby the output response signals fromthe transducers are selectively combined by the composite signal meanswithin the electronics package to form the composite signal for eachsaid receiver station, the composite signal being representative of aborehole-propagation mode.
 18. The receiver sonde adapted for reducingthe effects of noise on composite signals in accordance with claim 16,also comprising:an electronics package; wherein the transducers areattached to the chassis in paired combinations to provide a multi-modereceiver station, a paired combination being two transducers positionedon opposite sides of the chassis with each said transducer of the pairedcombination being equally spaced axially around the periphery of thechassis with respect to the other said transducers; and wherein thecomposite signal means is a remotely controllable composite signalmeans, whereby the output response signals are combined within theelectronics package to form the composite signal for each said receiverstation, the composite signal being representative of aborehole-propagation mode selected by the remotely controllablecomposite signal means.
 19. The receiver sonde adapted for reducing theeffects of noise on composite signals in accordance with claim 17wherein:each said transducer is a pressure sensitive, flat,piezoelectric transducer; the chassis includes a conductive metal; andthe common electrical ground is established by plating the inner face ofeach said piezoelectric transducer with a conductive electrode andbonding the inner face of each said piezoelectric transducer to theconductive metal of the chassis.
 20. The receiver sonde adapted forreducing the effects of noise on composite signals in accordance withclaim 19, also comprising:a charge amplifier for amplifying the outputresponse signal from each said transducer before the output responsesignal enters the composite signal means of the electronics package. 21.The receiver sonde adapted for reducing the effects of noise oncomposite signals in accordance with claim 19 wherein:each saidcompliant spacer that separates the receiver stations has approximatelythe same length whereby the receiver stations are longitudinallyseparated from each other by approximately the same distance; and eachsaid piezoelectric transducer is reduced in size such that the size ofeach said transducer is small with respect to the distance between thetransducers on separate said receiver stations, whereby, the transducersapproach a point source, so that, when the output response signals fromeach said receiver station are combined to form the composite signal, amore accurate indication of spatial resolution between the compositesignals is provided by the sonde.
 22. The receiver sonde adapted forreducing the effects of noise on composite signals in accordance withclaim 21, also comprising:a charge amplifier for amplifying the outputresponse signal from each said transducer before the output responsesignal enters the composite signal means of the electronics package. 23.The receiver sonde adapted for reducing the effects of noise oncomposite signals in accordance with claim 19, also comprising:an innerjacket is included in the semi-rigid receiver array, the inner jacketsurrounding the compliant spacers and the chassis and acting as abarrier between the liquid in the borehole and the interior of thesemi-rigid receiver array; and an insulating fluid within the interiorof the semi-rigid receiver array.
 24. The receiver sonde adapted forreducing the effects of noise on composite signals in accordance withclaim 23 wherein the inner jacket is an elastomeric boot.
 25. Thereceiver sonde adapted for reducing the effects of noise on compositesignals in accordance with claim 19 wherein each said transducer hasfrequency response characteristics to an input acoustic signal whichincludes a lowest resonant-response region which is above the highestfrequency within a selected operation bandwidth for the compositesignals, whereby, the transducer is functional to provide anapproximately constant ratio in amplitude and substantially little phasedifference between the output response signal from the transducer andthe input acoustic signal to the transducer, thus, the transducerproduces the output response signal to be a faithful representation ofthe input acoustic signal.
 26. The receiver sonde adapted for reducingthe effects noise on composite signals in accordance with claim 25wherein each said transducer has a lowest resonant frequency which ofeach said transducer is at least ten times greater than the highestfrequency within the selected operating bandwidth.
 27. The receiversonde adapted for reducing the effects of noise on composite signals inaccordance with claim 13, the sonde also comprising:the semi-rigidreceiver array also including non-receiving chassis; a firstnon-receiving chassis is positioned between the receiver stations andthe top bulkhead; a second non-receiving chassis is positioned betweenthe receiver stations and the bottom bulkhead; and wherein the compliantspacers separate the non-receiving chassis from the top bulkhead, thebottom bulkhead and the chassis, the compliant spacers being in contactwith the non-receiving chassis and maintaining the non-receiving chassisin an approximate fixed longitudinal position between the top bulkheadand the bottom bulkhead and enabling the receiver array to besemi-rigid, each said compliant spacer in contact with the non-receivingchassis forming another junction having a larger disparity in elasticproperties where each said compliant spacer contacts the non-receivingchassis.
 28. The receiver sonde adapted for reducing the effects ofnoise on composite signals in accordance with claim 27 wherein thecompliant spacers are annular rubber sleeves.
 29. The receiver sondeadapted for reducing the effects of noise on composite signals inaccordance with claim 27 wherein each said transducer is poled in thethickness expansion mode and each said transducer is attached to thechassis such that each said transducer is poled in the same radialdirection with respect to the center of the chassis.
 30. The receiversonde adapted for reducing the effects of noise on composite signals inaccordance with claim 29, also comprising:an electrical connectionbetween the receiver stations; wherein each said transducer has an innerface with respect to the chassis; wherein each said chassis has aconductive path, the inner face of each said transducer being connectedto the conductive path to establish a common electrical ground such thatthe output response signal from each said transducer attached to thechassis has a common polarity with respect to all output responsesignals of the chassis; and the electrical connection between thereceiver stations establishes a common electrical ground such that eachsaid composite signal has a common polarity with respect to the othercomposite signals.
 31. The receiver sonde adapted for reducing theeffects of noise on composite signals in accordance with claim 30, alsocomprising:an electronics package; and wherein the transducers areattached to the chassis in paired combinations to provide a multi-modereceiver station, a paired combination being two transducers positionedon opposite sides of the chassis with each said transducer of the pairedcombination being equally spaced axially around the periphery of thechassis with respect to the other said transducers, whereby the outputresponse signals from the transducers are selectively combined by thecomposite signal means within the electronics package to form thecomposite signal for each said receiver station, the composite signalbeing representative of a borehole-propagation mode.
 32. The receiversonde adapted for reducing the effects of noise on composite signals inaccordance with claim 30, also comprising:an electronics package;wherein the transducers are attached to the chassis in pairedcombinations to provide a multi-mode receiver station, a pairedcombination being two transducers positioned on opposite sides of thechassis with each said transducer of the paired combination beingequally spaced axially around the periphery of the chassis with respectto the other said transducers; and wherein the composite signal means isa remotely controllable composite signal means, whereby the outputresponse signals are combined within the electronics package to form thecomposite signal for each said receiver station, the composite signalbeing representative of a borehole-propagation mode selected by theremotely controllable composite signal means.
 33. The receiver sondeadapted for reducing the effects of noise on composite signals inaccordance with claim 31 wherein:each said transducer is a pressuresensitive, flat, piezoelectric transducer; the chassis includes aconductive metal; and the common electrical ground is established byplating the inner face of each said piezoelectric transducer with aconductive electrode and bonding the inner face of each saidpiezoelectric transducer to the conductive metal of the chassis.
 34. Thereceiver sonde adapted for reducing the effects of noise on compositesignals in accordance with claim 33, also comprising:a charge amplifierfor amplifying the output response signal from each said transducerbefore the output response signal enters the composite signal means ofthe electronics package.
 35. The receiver sonde adapted for reducing theeffects of noise on composite signals in accordance with claim 33wherein:each said compliant spacer which separates the receiver stationshas approximately the same length whereby the receiver stations arelongitudinally separated from each other by approximately the samedistance; each said piezoelectric transducer is reduced in size suchthat the size of each said transducer is small with respect to thedistance between the transducers on separate said receiver stations,whereby, the transducers approach a point source, so that, when theoutput response signals from each said receiver station are combined toform the composite signal, a more accurate indication of spatialresolution between the composite signals is provided by the sonde. 36.The receiver sonde adapted for reducing the effects of noise oncomposite signals in accordance with claim 35, also comprising:a chargeamplifier for amplifying the output response signal from each saidtransducer before the output response signal enters the composite signalmeans of the electronics package.
 37. The receiver sonde adapted forreducing the effects of noise on composite signals in accordance withclaim 33, also comprising:an inner jacket is included in the semi-rigidreceiver array, the inner jacket surrounding the compliant spacers andthe chassis and acting as a barrier between the liquid in the boreholeand the interior of the semi-rigid receiver array; and an insulatingfluid within the interior of the semi-rigid receiver array.
 38. Thereceiver sonde adapted for reducing the effects of noise on compositesignals in accordance with claim 37 wherein the inner jacket is anelastomeric boot.
 39. The receiver sonde adapted for reducing theeffects of noise on composite signals in accordance with claim 33wherein each said transducer has frequency response characteristics toan input acoustic signal which includes a lowest resonant-responseregion which is above the highest frequency within a selected operationbandwidth for the composite signals, whereby, the transducer isfunctional to provide an approximately constant ratio in amplitude andsubstantially little phase difference between the output response signalfrom the transducer and the input acoustic signal to the transducer,thus, the transducer produces the output response signal to be afaithful representation of the input acoustic signal.
 40. The receiversonde adapted for reducing the effects of noise on composite signals inaccordance with claim 39 wherein each said transducer has a lowestresonant frequency which is at least ten times greater than the highestfrequency within the selected operating bandwidth.
 41. A receiver sondeadapted for reducing the effects of noise on output response signalswhich are representative of borehole acoustic signals in a liquidcontaining borehole, the noise affecting the output response signals inthe borehole includes at least one of two types of noise, the two typesof noise being a road-noise and a tool-mode noise, the receiver sondecomprising:a top bulkhead and a bottom bulkhead; a semi-rigid receiverarray, the semi-rigid receiver array including at least one receiverstation and compliant spacers; each said receiver station having achassis with transducers attached thereon, each transducer of thetransducers being functional to produce the an output response signalfrom an input acoustic signal; and the compliant spacers are in contactwith and separate the chassis from the top bulkhead, the bottombulkhead, and each other, the compliant spacers maintaining the chassisin an approximately fixed longitudinal position between the top bulkheadand the bottom bulkhead and enabling the receiver array to besemi-rigid, each compliant spacer of the compliant spacers beingfunctional to decouple the noise propagating within the semi-rigidreceiver array by acting as a vibration isolator to absorb the noise atit propagates across each said compliant spacer and by forming ajunction having a large disparity in elastic properties where each saidcompliant spacer contacts the chassis, each said junction scattering thenoise to reduce the amount of noise which propagates across thejunction, whereby, the amount of noise that reaches the transducers isreduced, thereby, reducing the amount of noise that is coupled to eachoutput response signal.
 42. The receiver sonde adapted for reducing theeffects of noise on output response signals in accordance with claim 41wherein the compliant spacers are annular rubber sleeves.
 43. Thereceiver sonde adapted for reducing the effects of noise on outputresponse signals in accordance with claim 41, the sonde alsocomprising:the semi-rigid receiver array also including non-receivingchassis; a first non-receiving chassis is positioned between the chassisand the top bulkhead; a second non-receiving chassis is positionedbetween the chassis and the bottom bulkhead; and wherein the compliantspacers separate the non-receiving chassis from the top bulkhead, thebottom bulkhead and the chassis, the compliant spacers being in contactwith the non-receiving chassis and maintaining the non-receiving chassisin an approximate fixed longitudinal position between the top bulkheadand the bottom bulkhead and enabling the receiver array to besemi-rigid, each said compliant spacer in contact with the non-receivingchassis forming another junction having a larger disparity in elasticproperties where each said compliant spacer contacts the non-receivingchassis.